I. Field of the Invention
This invention relates to improvements in a process for the conversion of methanol to hydrocarbons, to improvements in a Fischer-Tropsch process for the production of hydrocarbons, and to improvements made in catalysts employed to conduct such processes. In particular, it relates to improved cobalt catalysts, and process for using such catalysts in the conversion of methanol, and Fischer-Tropsch synthesis to produce hydrocarbons, especially C.sub.10 + distillate fuels, and other valuable products.
II. Background
A need exists for the creation, development, and improvement of catalysts and processes, useful for the conversion of methanol and synthesis gases to hydrocarbons, especially high quality transportation fuels. Methane is available in large quantities, either as an undesirable by-product or off-gas from process units, or from oil and gas fields. The existence of large methane, natural gas reserves coupled with the need to produce premium grade transportation fuels, particularly middle distillate fuels, thus poses a major incentive for the development of new gas-to-liquids processes. However, whereas technology is available for the conversion of natural gas to methanol, in order to utilize this technology there is a need for new or improved catalysts, and processes suitable for the conversion of methanol to high quality transportation fuels, particularly middle distillate fuels.
The technology needed to convert natural gas, or methane, to synthesis gas is also well established. It is also known that synthesis gas can be converted to hydrocarbons via Fischer-Tropsch synthesis, though new or improved catalysts, and processes for carrying out Fischer-Tropsch reactions are much needed. Fischer-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen is well known in the technical and patent literature. Commercial units have also been operated, or are being operated in some parts of the world. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on Kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100Co:18ThO.sub.2 :200 Kieselguhr, parts by weight, and over the next few years by catalysts constituted to 100Co:18ThO.sub.2 :200 Kieselguhr and 100Co:5ThO.sub.2 :8MgO:200 Kieselguhr, respectively. The Group VIII non-noble metals, i.e., iron, cobalt, and nickel, have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates. Most commercial experience has been based on cobalt and iron catalysts. The cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughput. The iron catalysts, on the other hand, are not really suitable for natural gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2 ; with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O.
The need for a catalyst composition, and process useful for the conversion of methanol or synthesis gas at high conversion levels, and at high yields to premium grade transportation fuels, particularly without the production or excessive amounts of carbon dioxide, were met in large part by the novel catalyst compositions, and processes described in U.S. application Ser. Nos. 626,013; 626,023; and 626,026, filed June 29, 1984, by Payne and Mauldin; now U.S. Pat. Nos. 4,542,122, 4,595,703, and 4,556,752. The preferred catalysts therein described are characterized as particulate catalyst compositions constituted of a titania or titania-containing support, preferably a titania support having a rutile:anatase content of at least about 2:3, upon which there is dispersed a catalytically active amount of cobalt, or cobalt and thoria. These catalyst compositions possess good activity and stability and can be employed over long periods to produce hydrocarbons from methanol, or to synthesize hydrocarbons from carbon monoxide and hydrogen.
These cobalt-titania catalysts it was found, like most hydrocarbon synthesis catalysts, became coated during an "on-oil" run with a carbonaceous residue, i.e., coke, formed either during extended periods of operation or during feed or temperature upsets. The initially high activity of the catalysts declines during the operation due to the coke deposits thereon, and the operating temperature must be increased to maintain an acceptable level of conversion. Eventually the catalysts become deactivated to a point where the temperature required to maintain an acceptable conversion level causes excessive formation of methane and other light hydrocarbon gases at the expense of the desired C.sub.10.spsb.+ hydrocarbons, at which point it becomes necessary to regenerate, and reactivate the catalyst. Unlike many other catalysts commonly used by the refining industry however, when the coke deposits were burned from the cobalt-titania catalysts at oxidizing conditions by contact with air (or oxygen) at elevated temperatures, and the catalysts thereafter treated with hydrogen to reduce the cobalt metal component, the initially high activity of the cobalt-titania catalysts did not return to that of a fresh catalyst. Rather, their activity was considerably less than that of fresh cobalt-titania catalysts. Moreover, after the regeneration, and reactivation of the catalysts, there was no improvement in the rate of deactivation and the deactivation proceeded from a lower initial activity. This loss in the overall activity brought about by burning the coke from these catalysts at elevated temperatures in the presence of air (oxygen) is not only detrimental per se, but severely restricts the overall life of the catalyst, and threatens their full utilization in commercial operations.